Because it’s true!
The decision to break up is a subtle balance of forces between the desire to stay and the desire to leave.
Missing you means that a part of her still feels what was there.
She feels a connection but this connection is not strong enough to stop her from moving forward.
The decision to break up is rarely totally black or white.
Most women will have at least some mixed feelings about this.
It doesn’t stop them from eventually still taking the decision though… And going on with their lives.